Conventionally, as a technique for reading two-dimensional image patterns such as printings, photographs, or fingerprints or other patterns having slight height, there has been known an image reading apparatus which has a sensor array formed of opto-electric conversion elements arranged in matrix or in line, and by driving the sensor array to perform scanning, reads an image pattern of a medium which is placed in contact with a detection surface provided on the sensor array.
Such an image reading apparatus is now being greatly researched and developed in order to apply this technique to a so-called biometric authentication technique for identifying an individual by checking information such as a fingerprint specific to a biological body, since there has recently been a social demand for securities in management and protection of personal and corporate information or in crime prevention.
In a case where such an image reading apparatus is applied to a fingerprint reading apparatus or the like, one such apparatus starts the image reading operation when it is detected, as a motive to start the image reading operation, that a medium such as a finger is placed on a detection surface of its sensor array.
This type of image reading apparatus having a function for detecting placement of a medium generally comprises, as shown in FIG. 18, a sensor array 100p formed of a plurality of sensors 10p arranged in matrix, a medium detector 30p provided on the sensor array 100p, a detection circuit 40p for outputting a control signal upon detection of placement of a finger (medium) FG thereby to start the image reading operation, and a drive circuit 50p for driving the sensor array 100p when receiving the control signal.
However, this image reading apparatus has the following problems.
That is, in order to detect whether a medium is placed or not, the medium detector 30p and the detection circuit 40p are always put in an operated state. And a predetermined bias voltage is always applied to the sensor array 100p even while the image reading operation is not performed. As a result, the bias voltage is applied to the sensors 10p for a long time and the sensing property of the sensors 10p is gradually worn out.
Further, one end of the sensor array 100p is always grounded. Because of this, in a case where a medium is charged with static electricity, part of the static electricity might be discharged to the ground potential through the sensor array 100p. This might damage the sensors 10p and the drive circuit 50p, or cause malfunctioning.
Still further, according to this image reading apparatus, the image reading operation is started upon detection that a medium is placed, as described above. Once the image reading operation is started, the sensor array 100p continues to be driven at least until reading of the full image data is completed by this operation. Therefore, even in a case where the medium loses contact with the sensors 10p due to some cause in the middle of the image reading operation, the image reading operation is continuously performed, resulting in obtaining incomplete image data. This costs a fruitless operation and a time required therefor.
Yet further, let it be assumed that the image reading apparatus is used outdoors or in an environment where external light intensity is quite high. And let it be assumed that in this environment, the image reading operation is started with a finger (medium) FG placed on a detection surface 100s as shown in FIG. 19A, and after this, the finger (medium) FG is separated from the detection surface 100s as shown in FIG. 19B. In this case, if the image reading operation is continued as described above and if the sensors 10p are photo-sensors, the sensors 10p receive the external light having high intensity while they are driven. Due to this, an excess current flows through the sensors 10p, and the property of the sensors 10p might be greatly deteriorated.